1. Field of the Invention
The present invention relates to a lead outlet structure of secondary battery in sheet type. For example, the present invention relates to, it is not limited to them though, a lead outlet structure of a large capacity secondary battery in sheet type for an electric vehicle, a UPS (uninterrupted power supply) a road leveling and so on.
2. Description of the Related Art
One of secondary batteries that have higher capacity or more energy per volume or weight and have higher energy density is, for example, a lithium ion rechargeable battery that is a secondary battery with non-aqueous electrolyte, is composed of lithium or lithium alloy. This lithium ion battery has advantages such as no memory effect, little self-discharge and so on, therefore, it is widely used for video camera recorders, audio equipment, portable computers, mobile phones, various electric equipment, communication devices, optical equipment and audio equipment.
This lithium ion secondary battery generally composes of an electrode pair, a battery housing which encloses this electrode pair and seals inside with electrolyte, and a cathode lead and an anode lead those are connected from each of cathode electrode and anode electrode of the electrode pair to each of cathode terminal and anode terminal which are incorporated on the battery housing, where the electrode pair consists of sheets of cathode electrode that consists of a sheet of cathode collector and cathode active material coated on it, sheets of anode electrode that consists of a sheet of anode collector and anode active material coated on it and a separator that is stacked between them.
During charging, a lithium goes out of cathode active material and gets into electrolyte as an ion, then gets into anode active material, while during discharging, this lithium ion gets out of anode material and gets into electrolyte and goes back into cathode material.
Since it can accomplish its high energy density, lithium ion secondary battery like this is expected to use, for example, as a large capacity secondary battery for electric vehicle use. And a plenty of development and proposal have been conducted in this area so far.
Not only relatively small secondary batteries used for electric equipment, communication devices, optical equipment, audio devices and so on, but also larger secondary batteries for electric vehicles, are requested to be smaller in size, lighter in weight, thinner in thickness and free in shape.
Therefore, conventionally in regard of a battery housing, a flexible envelope form package of three layer laminated film that is composed of an inner layer of thermoplastic resin such as polyethylene or polypropylene which are stable with electrolyte and suitable for heat-sealing, a middle layer of metal foil that is flexible and strong such as aluminum foil and an external layer of insulating resin such as polyamide resin, which shows good electrical insulation.
Light, thin, and flexible lithium ion secondary batteries in sheet type are proposed to be obtained by inserting an internal electrode pair in sheet type and electrolyte into the above-mentioned envelope form package.
And as indicated in FIG. 5 and FIG. 6, this conventional lithium ion secondary battery in sheet type B consists of an internal electrode pair 1 in sheet type that is generally composed of cathode electrode 1a in sheet type, anode electrode 1b in sheet type and separator 1c stacked between them and a flexible envelope form package 2 that is made of laminated film which has an internal layer 2a of thermoplastic resin, a middle layer 2b of metal foil and an external layer 2c of insulating resin, which contains the above-mentioned internal electrode pair 1 and electrolyte inside. And also inside this envelope form package 2, each of cathode electrode la and each of anode electrode 1b are connected to a pair of cathode lead 3a and anode lead 3b, respectively. In addition, this pair of cathode lead 3a and anode lead 3b goes hermetically through heat sealed portion 4 of the envelope form package 2 and is fixed to this heat sealed portion. And the projected external portions of the cathode lead 3a and anode lead 3b out of the heat sealed portion 4 are used as terminals or external leads.
In this structure of electrode leads, the surface between the envelope from package 2, the cathode lead 3a and the anode lead 3b, which are extended, as external leads or terminals, to the outside of the envelope form package 2 is adhered by heat-sealing of inner layer 2a of thermoplastic resin of the envelope form package 2. And adhesion strength between these cathode lead 3a and anode lead 3b and an envelope form package 2 is not sufficient, especially the larger the capacity becomes, the heavier the cell becomes inevitably, consequently adhesion between a cathode lead 3a and an anode lead 3b, and an envelope form package 2 becomes unreliable with heat seal only. And if material of a cathode lead 3a is aluminum as in normal case, the adhesion strength on this aluminum cathode lead 3a becomes more unreliable, then heat sealed portion between the cathode lead 3a and the envelope form package 2 may have an opening and in some cases, water may go inside through this opening and may produce hydrogen fluoride, then the battery may deteriorate or the electrolyte may go out of the envelope form package 2.
Moreover, when capacity becomes larger and as a result large drain is desired, the section area of leads that is conducted from the inside electrode pair (cathode electrode and anode electrode) through the envelope form package to the outside is also required to be large. Then the larger this section area is, the more frequently the above-mentioned problems occur. For this reason a sheet type lithium ion battery in an envelope form package with capacity of more than 3 Ah is considered to be difficult to achieve, although it is expected to contribute to smaller or lighter equipment, machines, or automobiles with these lighter, thinner, flexible and versatile characteristics.